1. Field of the Invention
The invention relates generally to a method and the corresponding system for measuring the insertion loss (IL) of an optical component, and particularly to the method and the corresponding system characterized to eliminate the polarization dependent of such tested component and the testing instrumentation during the IL measurement.
2. The Prior Art
The Insertion loss (IL) of optical components used in a communication link defines the system's power budget and margin. Therefore, it is very important to be able to measure the IL of the optical components accurately before using them in the communication system. The IL of an optical component is defined as ##EQU1## wherein P.sub.in is the optical power entering the component and P.sub.out is the optical power emerging from the optical component. The standard measurement procedure is what is called cutback method. The conventional test setup to measure IL variation can be referred to Bellcore GR-326-CORE, issued Dec. 1, 1994, of which the corresponding equipment and setup are shown in FIG. 1.
This traditional method includes multiple light sources 1 connected to a coupler 3 via a 2.times.1 switch 2. The output port of the coupler 3 is connected to a 1.times.N switch 4 and one of the input ports of the coupler 3 is connected to an N.times.1 switch 7 for back reflection measurement. A chamber 5 is positioned between the switch 4 and the switch 7 with N switching ports thereof for receiving therein N devices 6 under testing wherein each device 6 is respectively connected to the corresponding ports of the switch 4 and of the switch 7. An optical power meter 8 is connected the other side of the switch 7 to display the measured value of the selected device 6. Understandably, the 2.times.1 switch 2 is intended to provide two choices of the light sources and the 1.times.N switch 4 and the N.times.1 switch 7 allow to measure different N devices in this measurement system. In this system, temperature change and humility change are intentionally applied in the chamber whereby the corresponding IL values under different conditions may be obtained from the power meter 8, thus deciding characters of IL variation of the tested device 6. In order to measure the PDL of the device under test, a polarization controller could be inserted between switch 2 and the coupler 3. The inserted polarization controller is intended to create some severe polarized conditions to see the resulting measured value of the device under testing under those situations.
Even though this measurement/testing system is designed to measure the IL as accurate as possible, there is an inherent disadvantage accompanied therewith which relates to the PDL (polarization dependent loss) and is almost inevitable. The PDL for a device or a system is the maximum, peak-to-peak insertion loss due to variation caused by a component when stimulated by all possible input polarization states, wherein ##EQU2## with dB unit and P.sub.max and P.sub.min are the maximum and minimum optical powers measured when all possible polarization states vary. The polarization dependent loss may be characterized to include polarization sensitivity, polarization dependent gain (PDG) or extinction ratio, etc..
When testing either passive or active optical systems with polarized light, the measurement result can be highly sensitive to the polarization dependent loss (PDL) of the individual components or the even testing instrumentation. This means that the IL of the device under test (DUT) may interferes with the PDL on the test and is substantially influenced thereby. It is helpful to measure the various parameters independently for a better understanding how each parameter affects the whole performance of the device or the system. It will not only benefit the device manufacturers but also the system designers. Further more, modern devices like integrated optics or polarizing devices, are sensitive to the input polarization in their measurement of the IL. Anyhow, careful characterizations are mandatory for a better understanding of the whole design, the limited factors and potential ways for improving the performance to the required level.
Generally, the IL measurement is very stringent in a long term (for example, 14 days) Bellcore reliability during the test, which suffers not only the IL variation interfering with the PDL, but also the coherent multiple reflections resulting from bad connections and causing optical interference. In fact, without intentionally actuating the polarization controller, the pure temperature/humility changes within the chamber 5 in the test may still create somewhat polarization thereof, thereby resulting in the mixed IL and PDL simultaneously which prohibits understanding the accurate IL of the tested device. In the Bellcore test as shown in FIG. 1, the influence of the IL mixing with the PDL could be classified with three different cases. In case (1), the DUT is under a good layout situation, e.g., good and firm connection between the parts, so that the IL variation itself is dominant and plus the minor PDL caused by the same temperature/humility change cycling. In case (2), the DUT is under a bad layout situation, e.g. bad connection between two parts, so that the IL variation will mix with the middle PDL caused by the polarization fluctuation derived from such bad connection. In case (3), the polarization controller 3 is in operation for a so-called PDL test mainly measuring the PDL of the DUT, so that the PDL variation is the dominant on the test result plus the IL variation due to the temperature/humility cycling. It should be understood that the aforementioned reflection points may include the bad fusion splices, bad connections, or bad fiber pigtail end-faces, etc.. Also, the aforementioned optical interference is also sensitive to the polarization states, and degrades the system performance and measurement accuracy.
Under this complicated situation, there are two traditional ways trying to remove the PDL uncertainty from the IL test result for assuring that the testing result only reflects the IL variation without any PDL variation involved therein. One is to use a broadband, unpolarized sources, such as ASEs (U.S. Pat. Nos. 5,345,331 and 5,337,375), SLDs (U.S. Pat. Nos.4,634,928 and 4,653,917) or LEDs. The other is to randomize the polarization states of the polarized laser (U.S. Pat. Nos. 5,408,545 and 5,359,678). The former is of the wider spectrum and thus makes it difficult to attribute a center wavelength to the single-wavelength loss measurement which is the earnest desired matter in the most tests, while the latter should be equipped with the expensive active components such as polarization generator/controller to generate all possible polarization states and averaging at the same time with a relative slow detector, which suffers extra contribution to adjust the rotation speed of the polarization generator and the samples averaging time of the receiver.
Therefore, it is strongly desired to have a method and the corresponding apparatus which may efficiently eliminate the undesired PDL from the testing system or testing result when the IL variation is tested while without the disadvantages of the prior arts.
Another object of the invention is to eliminate the PDL of not only the device under test of polarization dependency but also the testing equipment of polarization dependency, e.g. ..+-.0.05 dB for an optical spectrum analyzer or .+-.0.2 dB for an optical power meter.